A filling time, an initial pressure for clutch engagement and a gradient of build-up hydraulic pressure can be noted as evaluation parameters employable for a gear box. The filling time represents a period of time that elapses until an empty clutch pack is fully filled with hydraulic oil. Since no torque is transmitted for the period of filling time, it is desirable that the filling time is shortened as far as possible. If a clutch pressure is elevated to a high level when a clutch starts to be brought in an engaged state, shock caused by shift change may be amplified. Accordingly, it is desirous that the initial pressure usable for clutch engagement is reduced as far as possible. Since a property of acceleration and a riding comfort vary depending on a gradient of build-up hydraulic pressure after completion of the filling, it is necessary that the gradient is adequately determined depending on a variety of running conditions.
With a conventional gear box used for a construction machine, however, a mechanical type modulation valve is employed while utilizing a spring force. This modulation valve has a function of gradually increasing a hydraulic pressure active on a clutch in a purely mechanical manner when the clutch is brought in an engaged state, and this function causes the clutch pressure to be increased with a predetermined gradient.
However, a pattern indicative of a hydraulic pressure derived from this modulation valve is determined uniformly due to the structure specific to the modulation valve. Thus, with the conventional apparatus, the hydraulic pressure pattern is set to such an intermediate value that requirements for the above-described properties can satisfactorily be met to some extent.
However, the respective clutch properties as described above vary largely during shift change under various conditions such as a loading state, a time interval of shift change (representative of a period of time from the time when a clutch is turned off till the time when it is later turned on) and a speed stage at which the shift change has been effected. For the reason, as long as an uniform hydraulic pressure pattern is always provided like the conventional apparatus, there arises a problem that adequate shift change can not be achieved any more. If a command pressure pattern to be given is not correctly determined at the time of shift change, e.g., at the time of running of the vehicle on a slope under a large magnitude of load, at the time of an excavating operation or the like, a malfunction such as breathing or plunge-back (representative of a state that the vehicle moves back after it plunges into a mass of excavated gravel) occurs.
With respect to a problem of double engagement, a conventional transmission control system employs such a structure that a plurality of shift valves adapted to perform ON/OFF shifting for each clutch are connected in series to respective clutches so that when one clutch is brought in an engaged state, other clutch is released from the engaged state without fail so as to prevent double engagement from taking place with the clutches.
In recent years, a proposal has been made as to a shift change system wherein a plurality of (proportional) pressure control valves adapted to be actuated in response to an electrical command are connected to a plurality of shift change clutches in a parallel relationship. With this system, however, it is difficult that the respective pressure control valves are mechanically connected in series to the corresponding shift change clutches in a conventional manner so as to prevent double engagement from taking place with the clutches. Since it has been found that double engagement takes place with the clutches because of the presence of a drive circuit by way of which an electrical command is inputted into the pressure control valve or due to mechanical trouble with the clutch housing, it has been heretofore requested that a suitable process for preventing double engagement from taking place with the clutches is developed for the shift change system.
The present invention has been made with the foregoing background in mind and its object resides in providing an apparatus for controlling a gear box which assures that smooth shift change unit can always be achieved under various conditions of running of a vehicle.
Other object of the present invention is to provide an apparatus for controlling a gear box which assures that a shift change system in which each shift change clutch is individually controlled by an electronic type pressure control valve can reliably prevent double engagement from taking place with clutches.